The subject matter described herein relates generally to methods and systems for operating a wind turbine, and more particularly, to methods and systems for operating a control assembly for a wind turbine.
At least some known wind turbines include a tower and a nacelle mounted on the tower. A rotor is rotatably mounted to the nacelle and is coupled to an electric generator by a rotor shaft. In typical wind turbines, a plurality of blades extend from the rotor. The blades are oriented in a manner such that wind passing over the blades turns the rotor and rotates the rotor shaft, thereby driving the generator to generate electricity.
At least some known wind turbines are operated by a control system. Furthermore, at least some known control systems for wind turbines implement pitch control thereof by rotation of the rotor blades about a pitch axis. That is, these control systems are designed for regulating the rotor speed of the wind turbine by setting the angles of the blades, i.e., pitching the blades, with respect to the airflow. Pitching the blades for decreasing the rotor speed generally results in an increase of the load acting on some of the components of the wind turbine, such as the blades, the rotor, or the wind tower.
Generally, an increase of the speed of the wind impinging on the rotor blades causes an increase of the rotor speed. Under certain conditions, such as high winds in the area of the wind turbine, the rotor speed may eventually exceed a threshold value corresponding to the maximum allowable speed of the wind turbine (i.e., an overspeed).
At least some known control systems which implement pitch control are designed for monitoring the rotor speed by determining actual values thereof and aerodynamically decreasing the rotor speed (i.e., braking the rotor) by increasing the pitch angle of the blades as soon as an actual value of the rotor speed exceeds the maximum allowable speed of the wind turbine. In this situation, decreasing the rotor speed by pitching the blades may result in a particularly significant increase of the load acting on components of the wind turbine. Generally, such a significant load increase negatively influences the operating life of the turbine. In at least some known pitch control systems, the pitch control drives the rotor speed back under the maximum allowable speed of the wind turbine. At least some of these pitch control systems are designed to decrease the pitch angle as soon as the rotor speed is under the maximum allowable speed of the wind turbine for maintaining a high rotor speed but within the security margin of the wind turbine (i.e., below the maximum allowable speed of the wind turbine).
In such overspeed events regulated by known control systems, the increase and posterior decrease of the pitch angle generally result in alternating forces acting on the tower. In some cases, these alternating forces may excite the resonant modes of the tower and lead to a resonant vibration of the tower. Such a resonant vibration of the tower may require shutting down the wind turbine when the vibration exceeds a maximum allowable limit. A shutdown event generally implies a loss of the capacity for generating power by the wind turbine.
Accordingly, it is desirable to provide a method and/or apparatus capable of implementing a pitch control which avoids high load on the wind turbine components and/or diminishes the risk of a shutdown of the wind turbine due to an overspeed state of the wind turbine.